complexity & change in environment, biomedicine & society

A short response to a common error about heritability

Let me attempt a quick counter to the common error that a heritability of, say, 80% for height in humans means that the trait is strongly determined by genes that research is now uncovering and that the remaining 20% explains how average height can increase from one generation to the next [see quote below].

One way to estimate heritability involves comparing the similarity of identical twins, who share all their genes, with the similarity of fraternal twins, who share a smaller %; in both cases, the twins are raised together. Suppose I provide a height data set for twins of both kinds and heritability is calculated at 90%. Researchers seeing the 90% as genetic might plan to search for the height genes. The problem with that move is that in my data the twins included humans, snakes, and sorghum plants. So we wouldn’t expect the same genes to be involved in height from one pair to the next. Yet there is nothing in the data analysis to expect that even if the twins had all been humans.

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The text above is much longer than the 100 or so word norm for letters to the editor of New Scientist. So I probably failed in my attempt to distill the crucial idea into a letter-to-the-editor response to this quote from New Scientist 16 March 2013, p. 38:

a much better predictor of your height comes from looking at your immediate family. Genetics accounts for 80 per cent of the variation (Nature Genetics, vol 40, p 489), with over 50 gene variants that link to height found so far. The other 20 per cent is down to nutrition – particularly in the first two years of life – and whether the body had to divert energy to fight disease when it should have been growing. This 20 per cent largely explains why the species as a whole is getting taller. The average Dutchman, for example, was 16 centimetres taller in 1990 than in 1860 thanks to improvements in nutrition and healthcare. Still, the rate of growth in healthy, well-nourished Westerners has been slowing for decades, which suggests that there is a limit to how tall our genes will allow us to be under perfect growing conditions.